Sports such as Rugby League (RL) and Union (RU) are characterised by intermittent periods of high-intensity movement such as sprinting, jumping, and tackling, separated by periods of relatively low-intensity activity such as standing, walking, and jogging (Waldron et al., 2011a). In elite RL match play, athletes are required to run at 90-100 mmin-1 and execute repeated high-intensity efforts, which have the potential to delay recovery of neuromuscular function by up to 48 h (McLellan et al., 2011). This can be partly attributed to a rise in exercise induced muscle damage (eIMD) (Cheung et al., 2003). EIMD occurs in response to movements that involve repetitive eccentric or high-intensity contractions such as wrestling (Kraemer et al., 2001), and repeated sprints (Howatson and Milak, 2009), which cause disruption to sarcomere integrity (Clarkson and Hubal, 2002), elevate blood markers of muscle damage and inflammation, and impair the ability to produce force/torque (Warren et al., 1999). In addition, EIMD often leads to an increase in subjective pain following a latency period which typically peaks 24-48 h following injury (Cheung et al., 2003), a phenomenon known as delayed onset muscle soreness (DOMS) (Clarkson and Hubal, 2002).

In addition to the aforementioned physical demands and muscle damaging exercise, rugby athletes frequently engage in collisions with opponents, team mates and the playing surface. These collisions have been found to lead to decrements in performance (McLellan and Lovell, 2012), increased energy expenditure (Costello et al., 2018), and a rise in markers of muscle damage following match play (Lindsay et al., 2015b; Smart et al., 2008; Takarada, 2003). Moreover, collisions have been linked to soreness experienced in the hours and days following match play, that may persist throughout a competitive season (Fletcher et al., 2016). Indeed, an emerging body of research from combat sports such as mixed martial arts has linked impacts and collisions to a rise in muscle damage and inflammatory markers (Lindsay et al., 2016; 2017). Mixed martial arts can include a severe collision and impact component, occurring within a relatively confined area, and depending on the individual athlete's fighting style, may occur independently of high intensity exercise. These data suggest skeletal muscle ultrastructural damage results from blunt force trauma and may profoundly delay recovery. However, training and match play data provides insight into changes related to accumulated neuromuscular fatigue, and the aforementioned parameters are often used as criteria to evaluate individual changes in response to both EIMD and impact induced muscle damage (IIMD) (Lindsay et al., 2015b; Naughton et al., 2017). Therefore, the possibility of conflation of the effects of EIMD and IIMD exists. As such, the magnitude and time course of alterations in neuromuscular function and performance specifically resulting from IIMD remains to be elucidated. Identification of these potential changes may provide insight into the potential implications and subsequent management of IIMD (Naughton et al., 2017).

Research investigating the physiological effects and associated functional implications of IIMD in a controlled anner is limited. To date, studies have explored the effects of additional physical collisions in the context of a RL small-sided game (Johnston et al., 2014), intermittent-sprint protocol (Pointon and Duffield, 2012), and a team-sport conditioning circuit (Singh et al., 2011). These investigations have identified a significant attenuation in performance, and increases in indices of muscle damage, when compared to a non-contact condition. Collectively, these data suggest that the addition of physical collisions to exercise results in IIMD, an exacerbated decrement in muscle function, and consequently performance. However, the inclusion of high-intensity running and/or wrestling in these studies introduces eccentric muscle action and associated EIMD as a possible source of such changes (Howatson and Milak, 2009; Kraemer et al., 2001). Such exercise modalities have the potential to cause EIMD and may confound any conclusions specifically relating to the implications of IIMD. As such, whilst these studies have merit in examining impacts in somewhat of an ecologically valid manner, in the context of exploring IIMD considerable limitations exist. As we previously have highlighted (Naughton et al., 2017), to minimise conflation, research exploring IIMD should aim to do so in the absence of EIMD and possible muscle damaging exercise.

Therefore, the purpose of the present study was to capture and characterise the typical magnitude and duration of changes resulting from a standardised IIMD experimental protocol somewhat simulating collisions experienced during rugby match-play in athletes who are habitually exposed to blunt force trauma. The IIMD protocol was adapted to this population by exposing participants to a similar frequency, derived from published match-play research (Gabbett and Seibold, 2013), to which they typically experience.

Methods

Participants

Eighteen young, healthy men (Table 1) who regularly participate in contact sports such as RL or RU volunteered to participate in the present study. Participants had no recent history of lower-limb muscle, joint, bone or blood-related injury or health issue and provided written informed consent. The present study was approved by the University Human Research Ethics Committee. Participants were required to refrain from unaccustomed exercise or vigorous physical activity for the two weeks prior to testing and for the duration of participation. Similarly, participants were instructed not to use anti-inflammatory medication or engage in recovery strategies such as cold-water immersion or wear compression garments for the duration of the study. Finally, testing occurred during the off-season to ensure participants were not exposed to additional game or training related fatigue or muscle damage. The sample size (n=18) was derived from the mean and variance from two previous related studies (Johnston et al., 2014; Singh et al., 2011), and based on the effect size of 1, alpha level of 0.05 and power (1-0) of 0.80.

Experimental protocol

Participants were familiarised with testing procedures at least two weeks prior to testing (Vaile et al., 2008). During familiarisation, participants were familiarised with the questionnaire and all testing equipment and procedures, including the completion of four maximal 15 m sprints and four submaximal squat jumps separated by a 3 min recovery period. Participants were visually familiarised with the impact simulating equipment; however, no impacts were implemented prior to testing to ensure no muscle damage was introduced that may have influenced the study outcomes.

In the week prior to testing participants undertook Duel Energy X-ray Absorptiometry (DXA) to assess body composition. At the commencement of each testing session, participants had a venous blood sample collected for analysis of biochemical parameters. Thereafter, 15 m sprint and squat jump performance was assessed, and finally a recovery questionnaire was administered to assess perceived soreness. Following completion of baseline measures, the IIMD protocol was implemented. The same biochemical, performance and subjective measures were replicated immediately after the IIMD protocol, and again at 24, 48, and 72 h thereafter (Figure 1).

Total body bone mineral density, fat mass and lean tissue mass were assessed. The DXA was calibrated using phantoms as per the manufacturer's guidelines on each testing day prior to measurement. Participants wore minimal clothing and were positioned centrally on the scanning bed with foam hand and feet positioning aids. All tests were conducted by the same experienced and licenced operator. The scans were analysed automatically using GE enCORE v.13 software (GE Healthcare, UK) with the Geelong reference database and regions of interest subsequently confirmed by the operator. Whole body composition data was included for analysis.

Blood markers

Venous blood samples were collected at the beginning of each testing time-point. Each sample (5-8 mL) was collected from the superficial antecubital vein using standard venepuncture techniques. All samples were collected into serum separator tubes (SSI, Victoria, Australia) allowed to clot, and the serum separated at 1200 rcf (centrifugal...